The temperature of the waters of the North Atlantic Ocean can be used as a climate indicator to predict, up to three months in advance, periods of extreme events linked to the reduction of rains and intense droughts in the Northeast region of Brazil. This is one of the main conclusions of a study published in the scientific journal Geophysical Research Letters, which involved researchers from Brazil, China, Australia and Germany.
The group applied a new methodological approach, focusing on the rainfall deficit, and showed that, in recent years, the influence of the North Atlantic has become more persistent than the performance of the tropical Pacific, hitherto identified as one of the impact factors on the intensity of droughts in the Northeast. At the same time, the connection between the Pacific and the North Atlantic became more frequent, suggesting that these interactions between tropical ocean basins have reinforced droughts in the region in recent decades.
“The work was motivated by the great drought registered between 2012 and 2015. This long period made us reflect, from a meteorological point of view, how the temperatures of the tropical oceans influence these climatic conditions. The difference now is the innovative methodology, which explores the issue of the different areas of the Pacific and Atlantic and the drought pattern in the Northeast. These results serve as a management tool for meteorological centers to forecast events with this potential in advance”, says to Agência Fapesp Lincoln Muniz Alves, a scientist at the Instituto National Space Research Institute (INPE) and one of the authors of the article.
The study was supported by Fapesp through a Thematic Project linked to the National Institute of Science and Technology for Climate Change and whose responsible researcher is Professor Elbert Einstein Nehrer, from the Federal University of São Paulo (Unifesp).
The drought that hit the semi-arid region of the Northeast between 2012 and 2015 had record intensity and impact, destroying agricultural areas, leading to a lack of water and affecting cities and small towns. Other works have already pointed out changes in atmospheric circulation as causes of this situation, suggesting an active role for warmer than normal surface waters in the Atlantic Ocean. El Niño, a weather phenomenon that involves an unusual warming of the Pacific, also contributed to the worsening of the situation.
At the time, El Niño was considered one of those with the greatest impact (after those recorded in 1982-1983 and 1997-1998), causing losses in different regions of the world. In Brazil, there was intense drought in the Northeast and in the Amazon, prolonged drought in the North, in the center-north of Minas, Goiás and the Federal District, in addition to floods in the South.
“This type of El Niño, called ‘canonical’, that is, with the heating pattern in the same region of the Pacific Ocean, has changed both its position and intensity. At the same time, we have seen anomalous warming in the Atlantic in recent decades. From the mix of analyzes carried out, the article provides support so that those who work with forecast can look, months in advance, at the signals coming from the tropical Atlantic. The Pacific influences, but the Atlantic has greater weight”, completes Alves.
New parameters
The purpose of the study is to use nonlinear phase coherence methods and generalized event synchronization analysis to understand the mechanisms of cause and effect. For this, the scientists interpreted the relationships between sea surface temperature variability (SST) and the standard precipitation index as direct interactions, while the relationships between oceans were evaluated as indirect effects on rainfall.
The researchers used precipitation data from the Climate Prediction Center, a US federal agency that is part of the National Weather Service of the National Oceanic and Atmospheric Administration (NOAA). Four regions were selected: the Brazilian Northeast, which has been the center of drought in recent decades; the area called Niño 3, where there was intense activity of the ENSO phenomenon (El Niño – Southern Oscillation); the North Atlantic and the South Atlantic, both similar areas used in previous works.
To verify consistency, the results were compared with the so-called Niño 4 region, which includes the Central Pacific and the extended South Atlantic. For each domain, the spatial mean of the variable of interest and the daily anomalies were calculated based on the period 1981-2010, with the rainy seasons defined from January to April, and the dry seasons from May to August.
The results revealed a dominant role of the North Atlantic for the deficit of precipitation and droughts, particularly in the last decades. In addition, precipitation frequencies and sea surface temperature changed after very strong El Niño and La Niña events, resulting in a higher probability of phase coherence.
“There is no longer a normal or linear pattern, as it was observed three decades ago. Several other studies have corroborated the result we obtained. This methodology reveals that there is no linear pattern to build forecasts. The research shows that it is necessary to conventional and highlights the importance of looking at other areas of the oceans, not just focusing on the Pacific”, says Alves.
Among the conclusions of the article, the group also points out that other factors, such as changes in land use, can lead to changes in the hydrological cycle, as already demonstrated in modeling studies, particularly on the Amazon basin. Therefore, the scientists suggest that further work with the developed methodology can focus on how these changes in land use alter climate characteristics and interactions.
“When we discuss climate variations, we are also talking about socioeconomic impacts and biodiversity. Therefore, meteorological centers can use the model to work on prevention, focusing on public policies or decision-making on mitigation actions for extreme events”, completes Alves.
The article Phase Coherence Between Surrounding Oceans Enhances Precipitation Shortages in Northeast Brazil can be read here.
